One of the most common uses of series-connected light strings, particularly of the commonly called “miniature” type, is for decoration and display purposes, particularly during Christmas time and other holidays. Such light strings are especially popular for the decoration of Christmas trees, both inside and outside commercial, industrial and residential buildings, trees and shrubbery, and the like.
Probably the most popular light set currently available on the U.S. market comprises one or more strings of 50 miniature light bulbs each. In Europe and other countries that have 240-volt power rather than 120 volts, strings of 100 lights are more common. Each bulb typically has an operating voltage of 2.5 volts, and the filaments of each 50-bulb string are connected in an electrical series circuit arrangement. If overall light sets of more than 50 bulbs are desired, the common practice is to provide a plurality of 50-bulb strings, with the bulbs in each string connected in electrical series, and with the plurality of strings being connected in a parallel circuit arrangement with respect to each other to form the light set.
As the bulbs in each string are connected in series, when a single bulb fails to illuminate for any reason, the whole string fails to light, and it is very frustrating and time consuming to locate and replace a defective bulb or bulbs. Usually many bulbs have to be checked in order to find the failed bulb. In fact, in many instances, the frustration and time-consuming efforts are so great as to cause one to completely discard and replace the string with a new string. Replacement, however, does not offer a practical solution if the lights are on an already decorated Christmas tree where removal could cause damage to the ornaments, or on wire frame yard decorations where the lights have many clips and wire ties holding them to the frame.
Light bulb manufacturers have also attempted to solve the problem of light section or string failures caused by single bulb failure by designing each light bulb in the string in a manner whereby the filament in each light bulb is shorted by a shunting device whenever it burns out for any reason, thereby preventing an open circuit condition from occurring in the socket of the burned-out bulb. However, in actual practice, it has been found that such short circuiting feature within the bulb does not always operate in the manner intended, resulting in the entire series section or string going out whenever a single bulb burns out.
The above-cited attempts to prevent string failure in series-circuit light strings have included the use of one or more shunt devices in association with each bulb and socket combination. The shunt is typically positioned directly within the glass envelope of each bulb in the string, making the effectiveness of the shunt depend on the presence at all times of a bulb within each of the bulb sockets in the string. In operation, the shunt provides an alternate path through which electric current will flow in the event of filament failure. After bulb failure and as long as the bulb remains in the string, the shunt allows current to continue to flow through the bulb, thereby preventing the failure of the entire series section of the light string.
The shunt is typically made at least in part of a material that is non-conductive as long as the bulb filament is operative, but becomes conductive whenever the filament fails. In normal operation, current will flow through the filament to produce incandescent light within the bulb envelope. When the filament breaks, however, the increased voltage differential across the bulb lead wires causes the non-conductive material to break down so that current continues to flow through the shunt in the failed bulb to the other bulbs in series therewith. That is, each shunt is inoperative unless and until it is subjected to substantially the full output voltage of the power source. When the bulb filament associated with a shunt fails, that shunt is subjected to the full voltage of the power supply, which renders that shunt operative to bypass the failed filament. A variety of different shunt structures and materials are well known in the industry, such as those described in U.S. Pat. Nos. 4,340,841 and 4,808,885.
In actual practice, the shunts described above do not function with a high degree of reliability, and thus the failure of a single bulb still often extinguishes the entire string.